It is hardly possible to determine the velocity of the blood's motion in the healthy state ; for individuals differ from each other in this respect, and considera ble variety probably takes place in diffe rent parts of the body. It is generally supposed, that the blood flows in a more gentle stream through the small arteries than in the arterial trunks ; and that the velocity of its current is somewhat less in the veins than in the arteries of the body. These differences have, however, been exaggerated by former physiologists. The mean velocity of the blood in the aorta is calculated at eight inches for each pulsa tion, which gives about fifty feet in a minute. If we reflect, that the systole of the ventricle, which gives the whole im pulse to the blood, occupies only one third of the whole pulse, the velocity of the blood's motion must be trebled in that division of the time. It is said that this velocity, which we have assigned to the blood's motion at its departure from the heart, becomes speedily diminished in its further progress ; and the diminution has been deduced from various causes. The first and most powerful of these is the constantly increasing area of the branches, when compared with the trunk of an artery. (See ANATOMY.) It is a well-known law in hydraulics, that the velocity of a fluid passing through an in verted cone constantly decreases, and that the diminution of velocity is in the ratio of the increase of area. The mathe matical physiologists have also, noticed the effects of friction ; deducing these from a comparison with the course of fluids in dead tubes. Other causes have been derived from the same source ; hence the serpentine course of some arteries, the unfavourable angles by which they some times arise, and their communications with each other, are enumerated among the cir cumstances which retard the course of the arterial blood. But it must be remember ed, that in viewing these retarding cau ses we are considering their action on the blood, as if this fluid were contained in inanimate tubes, and influenced merely by the contraction of the heart, without taking into the account any accessory impulse, which may be, and probably is, derived from the arteries. This retarda tion has been variously estimated by dif ferent calculators, who have all made it very considerable. Hales supposes the blood to flow through the capillary arte ries of a frog at the rate of two-thirds of an inch in a minute, which will be about 650 times slower than in the human aorta. Robinson and Whytt have gone still further : the former stating, that the velocity of the blood's motion in the aor ta is to that in the smallest vessels as 1100 to 1. We mention these calcula, tions, to spew what absurdities have been committed by men of the greatest abili ties, when they have applied the laws which regulate the properties of dead, matter to the living functions of the ani mal machine. llaller's observations on the circulation in living animals, (Elem. Phys. lib. vi. sect. 1. §. o0) entirely over throw these calculations. He found by his microscopical experiments, that the blood flowed generally as rapidly through the small as through the larger vessels. He states also, that in living animals it is poured out as far from a small as from a large artery. The numerous and diver sified experiments of Spallanzani afford additional evidence of the same truth.
We have stated, that the blood is thrown into the arteries by separate con tractions of the heart ; yet these vessels are constantly full, as may be proved by opening them during the heart's diastole. For the. blood flows on in such a way, that the subsequent quantity discharged from the right ventricle, overtakes that which is before, and thus causes the pul sation of the arteries. The excess of ve
locity in the blood coming from the heart, over that contained in the arteries, be comes constantly less ; and at a certain point ceases altogether. Here the pulse ceases also : hence in microscopical ob servations on the course of the blood in small vessels, its stream appears to be uniform; and it is commonly stated, that the pulsation ceases in vessels of about one sixth of a line in diameter.
The motion of the blood in the minute veins seems to be equal to its velocity in the small arteries; this increases in the larger trunks; and there is a constant ac celeration in the blood's course until it arrives at the heart. This fluid is passing through tubes which constantly decrease in area; and it follows of necessity, that by diminishing the channel of a fluid, its course must be accelerated. Hence the trunks of the venze cavx return to the heart, within a given time, as much blood as the aorta carried out of this viscus.
The motion of the blood along the veins must be derived from the impulse which it receives from the heart, and from the actiof (if there be any) of the arteries. Its circulation in these vessels is aided by the contraction of the muscles, which must urge on the contained fluid towards the heart; since their valves prevent any retrograde motion.
We shall readily perceive, that no cer tain calculation can be formed of the powers of the heart, when we consider that neither the quantity of blood ex pelled at one pulsation, nor the distance through which it passes in a given time, • • nor the velocity of its course, can be de , lined with any certainty ; much less can we form any accurate estimate of the ob stacles which occur to the blood's motion, which must considerably affect such a calculation. We may however approach in some degree to the truth, by collecting and comparing the results of probable conjecture. If we calculate the blood contained in the body at thirty pounds, the number of pulsations in one minute at seventy-five, and the quantity expelled from the left ventricle at each pulsation at two ounces and a half, the whole quan tity will pass through the heart about twenty-two times in the course of an hour ; and it will perform the circulation once in less than three minutes. The velocity with which this blood is pro pelled by the systole of the left ventricle may be collected from the violence with which it is ejected from a wounded artery, and the altitude to which it as cends. Blumenbach has seen it projected more than five feet from the carotid of an adult during the first contractions of the heart. Our countryman Hales calculated from his experiments, in which he mea sured the height of the blood's ascent in a glass tube, inserted into a large artery, that it would be thrown seven feet and a half from the human carotid: he estimates the surface of the ventricle at fiftee4 square inches; and thus finds that one thousand three hundred and fifty cubic inches, or about fifty-one pounds weight, press upon the left ventricle, and must be overcome by its systole. Many other calculations of the powers of the heart have been formed upon mathematical principles; but different persons have been led to such opposite results, that we are warranted from this circumstance in disregarding them altogether. Borelli makes the powers of the heart equal to an hundred and eighty thousand pounds; Keill to eight ounces. Sense observes, that if s weight of fifty pound be attached to the foot, with the knee of that side placed on the opposite one, the weight will be elevated at each pulsation : this weight is placed at a considerable dis tance from the centre of motion ; and, allowing for this circumstance, he esti mates the moving power at four hundred pound.